Experts say IT hiring activity is the highest it has been in a long time. For example, the Dice.com Web site says its listing of tech jobs is up 30% compared to last year to a total of more than 76,000 jobs, with the hottest areas being mobile application development and cloud computing.

The level of hiring activity "is about as close to 1999/2000 as I've seen in a long time," says Kathy Harris, managing director of Harris Allied, a New York City IT executive search firm. "It's very, very active."

CIOs may not realize how vulnerable they are to losing their best workers to these new opportunities.

"Salaries have been flat for a long time. IT budgets have been cut. Positions have gone unfilled, and people had to do extra work. You have a lot of disgruntled people out there who have been looking quietly for about a year, and now you're seeing a lot of hiring activity and a lot of opportunities out there," explains Alice Hill, managing director of Dice.com. "Employers have to be very vigilant in looking for signs that they are being poached."

In a February e-mail survey of 2,700 tech hiring managers, Dice.com found that 54% anticipate tech poaching will become more aggressive in 2011. Among the locales where tech poaching is the biggest threat are Silicon Valley, New York City and Dallas.

"There has been a lot of angel funding for startups. People who are toiling away in a bigger company, are now getting offered some interesting work to create what might be the next Facebook," Hill says.

For CIOs, here are a few signs that your employees may be job hunting:

1. They're taking single days or half-days off. And they're coming to work more formally dressed than usual.

2 They're getting current on their expense accounts.

3 They've updated their LinkedIn profiles, adding connections and recommendations from former colleagues and bosses.

4. They've posted their resumes on job boards.

5. They are making negative comments about the company on their personal Twitter or Facebook pages.

Companies that have suffered from weak financial returns are most susceptible to poaching of tech talent. That's because employees in these companies often feel undervalued, unappreciated, and as if there is no opportunity for advancement.

"When recruiters are calling people, they're probably going to focus on whatever the weakness is of the organization," Harris says. "They're going to focus on the business conditions ... if the company is going through a hard time, if employee attrition is bad or if there is no opportunity for professional growth with that company."

CIOs need to be especially on the alert for tech poaching if they've already started to lose key employees to rivals.

"When your people start to leave, it's a slippery slope," Harris says. "Really good people are looking to work with high-quality colleagues. They want to work with the best people. So if the boss that they thought highly of has moved on, they'll be worrying about the opportunity to learn and grow and be mentored by something that is just as great."

Hill says that to retain key employees, CIOs don't necessarily have to raise salaries. Techies are more interested in perks such as flexible schedules, regular telecommuting and training in an emerging technology such as mobile application development than they are in money.

"For employees, it is a good time to go to your boss and ask for some of those perks so you don't have to jump ship," Hill says, adding that salaries will remain relatively flat even for those who change employers. "You're not going to see big jumps in pay even if you leave."

For CIOs, the best strategy is to have a retention plan for star employees and to promote from within.

"Employee engagement, talent retention, needs to be a priority. You need to have a plan that involves technology training and development," Harris says. "I'm a huge advocate of tech training for folks. If a new technology is brought in, and the organization goes out and hires a new person instead of training somebody internally, that says to the others: You are in a dead-end job. ... Look first to your own people to fill internal jobs."